Use hgpath instead of "hg" to fix --HEAD hg builds
Use hgpath instead of "hg" to fix --HEAD hg builds

Fixes #3628.
With environment filtering, hg was no longer in the path,
so make sure to provide the path when calling hg commands.
last_commit was the source of the problem in #3628
but I fixed source_modified_time as well.
I didn't see any others with this problem.

Linuxbrew/brew,konqui/brew,MikeMcQuaid/brew,EricFromCanada/brew,vitorgalvao/brew,DomT4/brew,alyssais/brew,maxim-belkin/brew,Linuxbrew/brew,mgrimes/brew,ilovezfs/brew,reelsense/brew,JCount/brew,EricFromCanada/brew,vitorgalvao/brew,reitermarkus/brew,Linuxbrew/brew,reelsense/brew,reelsense/brew,nandub/brew,alyssais/brew,JCount/brew,MikeMcQuaid/brew,hanxue/linuxbrew,ilovezfs/brew,mgrimes/brew,DomT4/brew,claui/brew,pseudocody/brew,mahori/brew,claui/brew,claui/brew,hanxue/linuxbrew,nandub/brew,Homebrew/brew,sjackman/homebrew,hanxue/linuxbrew,mahori/brew,JCount/brew,vitorgalvao/brew,Linuxbrew/brew,mahori/brew,MikeMcQuaid/brew,EricFromCanada/brew,reitermarkus/brew,mgrimes/brew,DomT4/brew,claui/brew,ilovezfs/brew,alyssais/brew,pseudocody/brew,EricFromCanada/brew,Homebrew/brew,nandub/brew,maxim-belkin/brew,reitermarkus/brew,DomT4/brew,konqui/brew,MikeMcQuaid/brew,JCount/brew,pseudocody/brew,sjackman/homebrew,sjackman/homebrew,konqui/brew,nandub/brew,sjackman/homebrew,maxim-belkin/brew,vitorgalvao/brew,mahori/brew,Homebrew/brew,konqui/brew,reitermarkus/brew,Homebrew/brew